The present invention relates to a protection switching system for a carrier transmission line, more particularly it relates to a method to increase the reliability of the protection system and to utilizing a stand by system as a signal transmission line to economize the carrier transmission system.
It is essential to provide a stand by system to increase the reliability of a carrier transmission system, and when some trouble occurs in the working line, the signal is switched to a stand by system to transmit the signals. Such switching is used very commonly for wireless transmission lines, since fading is inevitable in radio transmission, and therefore, frequency diversity and space diversity systems have been developed. But the situation for a wired transmission line system is slightly different, since there is usually no need to be concerned about fading. The system reliability has been increased by increasing the reliability of equipment or the wiring line. But it is still necessary to provide a stand by system or protection switching system to attain high reliability for wired communication systems.
In high reliability communication systems there occurs other problems, one of which is to assure the reliability of the stand by system or protection switching system itself, even if the high reliability of the total system can not be attained. One approach has been to provide a parallel system for a signal. For example, at a transmitting terminal the signal is branched to form three identical signals and transmitted through three parallel lines. At the receiving terminal, the three received signals are compared with each other and if a difference between them is found, then a majority law is adopted. Namely, if two of the three signals are equal to each other, then the third is rejected as being in error.
Such a system is reliable but expensive, so it is used only in limited situations where very high reliability is required. On the other hand, the reliability of equipment has been improved to a great extent and it has become practical to provide a single stand by system for several transmission lines, though there remains the problem of the reliability of the stand by system. Moreover, a modern multi-channel transmission system, such as a light communication system, is expensive and it has become too expensive to use the stand by system only for the transmission of auxiliary signals such as service or maintenance signals between the stations.
Another problem of protection switching systems is reducing the time for switching the signal from a working line to a stand by line when trouble occurs, because the transmission is interrupted during this switching time. A "hitless" system in which even an instantaneous breakdown of the transmission line is not allowed, is desirable for modern data transmission lines.
Before disclosing the present invention, a prior art protection switching system for a carrier transmission line will be briefly described. FIG. 1 is a block diagram of an exemplary prior art digital multiplex signal transmission line system comprising a protection switching system. Multiplexed telephone, television (TV) or data signals, SYS-1, SYS-2, . . . , are transmitted between a terminal station A and a terminal station B (they are called hereinafter STATION A and STATION B, respectively) via the transmission lines 11, 12, 13, . . . . The transmission line may be a cable, optical fiber cable or any other type, and contains signal lines in both directions, i.e., up (from STATION B to STATION A) and down (from STATION A to STATION B) signal lines. In FIG. 1, the signal of SYS-1 is transmitted through a transmission line 12, and the signal of SYS-2 is transmitted through the line 13. The switching of these signals are done in line protection (LP) switches 1 and 2 controlled by channel switching controllers 3 and 4 in stations A and B, respectively.
The transmission line 11 is a service line or stand by line, through (or over) which is usually transmitted a service signal between the stations, such as a telephone order wire signal for the maintenance service men between the stations, SC signal (supervisory and control signal) and SV signal (supervisory or surveillance signal, etc., that is used for service and order between the stations). Such signals are called auxiliary signals. Sometimes the system is provided with a redundant SC signal line (not shown) between the switch controllers 3 and 4 of each station.
In the stand by line 11 are inserted in the downward direction (from STATION A to STATION B) a terminal repeater (REP) 7-1-1, repeaters 8-1-1, 9-1-1 and a terminal repeater 10-1-1. In the upward direction (from STATION B to STATION A) are inserted a terminal repeater 10-1-2, repeaters, 9-1-2, 8-1-2 and a terminal repeater 7-1-2. They all work together for relaying the signals. The configuration of equipment is similar for the working lines 12, 13 and so on. Usually, a terminal repeater includes a multiplexer, demultiplexer, etc. and sometimes it includes an electro-optical converter for light transmission systems. Though not shown in FIG. 1, the transmission line may include additional repeaters, amplifiers or repeater stations.
The reference numerals 5 and 6 designate dummy signal generators which send dummy signals over respective stand by lines. The dummy signal is detected by the terminal repeater of the opposite station in order to monitor the transmission characteristics of the stand by line. Such dummy signals are especially effective for optical fiber cable systems to maintain the repeaters at their predetermined condition, since the time constant of AGC (automatic gain control) for optical communication equipment is usually long compared to that of electronic communication systems.
Each receiving terminal repeaters 10-1-1, 10-2-1, 10-3-1, 7-1-2, 7-2-2, 7-3-2, etc. is provided with detectors to watch the input signal level, coding error, bit error or trouble in equipment, so that if trouble or an abnormal situation is detected, the terminal repeater (REP) sends an alarm to the respective channel switching controller 3 or 4. Then the channel is switched by the LP switch 1 or 2 under the control of the channel switching controller 3 or 4. For example if the REP 10-2-1 in the STATION B detects an abnormal condition, that means a problem occurred in some part of the down signal line in transmission line 12 (that is, in the transmission system from REP 7-2-1 through line 12 to REP 10-2-1) and an alarm is sent to the channel switching control (CSC) 4. The CSC 4 sends a command to the CSC 3 in the STATION A to switch the working line 12 to the stand by line 11. This command is sent through the stand by line 11 or redundant SC signal line (not shown). In some recent systems the command is sent via upward working lines 11, 13 and so on using their idle channel or idle bits.
Receiving the command, the CSC 3 in the STATION A controls the LP switch 1 to switch the signals SYS-1 from line 12 to line 11, and when the switching is completed, the CSC 3 sends a signal indicating that the switching is completed to the CSC 4, via the redundant SC signal line (not shown) or idle channel or idle bits of downward working lines (lines 11, 13 and so on). When the signal is received by the CSC 4, it controls the LP switch 2 to switch the main signal SYS-1 from the line 12 to the stand by line 11.
Thus the system switching is finished. Then the dummy signal generators 5 and 6 send dummy signals through transmission line 12 by the command of CSC 3 and 4, and the rehabilitation work on line 12 is monitored by checking the received dummy signal at the REP 10-2-1 or 7-2-2.
Generally, a single stand by line is enough for several working lines. But, depending on the reliability required of the system, the number of stand by lines may be increased. Also, usually, when the failed line (line 12 in this example) is repaired, the system is again switched to the former line 12, and the line 11 is reserved as a stand by line. Such a system is called a fixed protection line system. On the contrary, a system called a floating protection line system keeps the restored line (line 12 in this example) as a new stand by line, and uses the former stand by line (line 11) as a working line. Usually, the fixed protection line system is used.
As mentioned before, one of the problems of the prior art is the reliability of the stand by system; if the stand by system has failed, the system cannot work. Other problems will be described with reference to FIGS. 2 and 3. In FIGS. 2 and 3, like reference numerals and characters designate like or similar parts of FIG. 1.
FIG. 2 is a block diagram showing a main part of a prior art switching system. The figure shows only the downward lines. In practice, a similar circuit for the upward lines is used forming pairs with the downward lines. The following explanation will be given assuming a single stand by and fixed protection line system. The switching elements 1-1, 1-2, 1-3, - - - and 2-1, 2-2, 2-3, - - - are composed of a mercury relay switch, for example. Resistors R.sub.1, R.sub.2, R.sub.3, - - - have the same resistance as the line impedance (usually 75 ohms) to terminate the transmission line. In the example of FIG. 2, the dummy signal generated by the dummy signal generator 5 is transmitted from the STATION A to STATION B via the line 11 (the stand by line), and terminated by the resistor R.sub.1.
If trouble occurs in the working line 12, the signal SYS-1 is switched to the stand by line 11 in the manner described before. In this case the contact points of the switching elements 1-1, 1-2, 2-1 and 2-2 shift from the position shown by solid lines to the position shown by broken lines, in response to commands from the CSC 3 and 4. Then, as can be seen in FIG. 2, the signal SYS-1 is switched from transmission line 12 to 11, and the dummy signal is switched from the stand by line 11 to the failed line 12 and terminated by the resistor R.sub.2. By analyzing the dummy signal received by REP 10-2-1, the fault point is detected and repaired.
When the trouble is removed, the working line is shifted to the line 12 and the line 11 is kept again as a stand by line. In such a system, the terminating resistors R.sub.1, R.sub.2 and so on are part of the switching elements 2-1, 2-2 and so on. Therefore the signal arrived at the STATION B via the stand by line, e.g., line 11, cannot be output from the switching elements. So it has not been possible to use the stand by line 11 as a signal transmission line.
Recently, the transmission line system has become increasingly expensive as it becomes highly multiplexed and complicated. In such a system, the stand by system is almost kept in an idle station, so long as trouble does not occur and a working line is not switched to the stand by line. There arose a desire, therefore, to utilize the stand by line as a working line. Moreover, as can be seen in FIG. 2, the switching elements for the STATION A (1-1, 1-2 etc.) must be a different type from that of STATION B (2-1, 2-2 etc.). It is not desirable to provide and stock different types of parts from the viewpoint of economy.
Another problem of the prior art is the switching time. FIGS. 3(a) and 3(b) are time charts showing the sequence of the protection line switching in a prior art system. FIG. 3(a) shows a normal sequence of the switching and FIG. 3(b) shows the sequence of the system when the switching has failed by accident. In FIGS. 3(a) and 3(b), time flow is shown from top to bottom, and the thick vertical lines shows switching which is performed in STATION A and STATION B.
Referring to FIGS. 1 and 3, if the terminal repeater REP 7-2-2 in STATION A detects an abnormal condition and sends an alarm signal to the CSC 3, the alarm signal triggers the switching of the line 12 to the line 11. As mentioned before, the switching command is sent from the STATION A to the STATION B and it takes about 5 ms for the command to reach to the STATION B. Then, the CSC 4 commands the LP switch 2 to switch the line 12 to the line 11. It takes about 2 ms for this switching. When the switching in STATION B is completed, the CSC 4 sends a status signal to the STATION A that the switching is completed. It again takes about 5 ms for the status signal to reach to the STATION A. After receiving this signal, the CSC 3 commands the LP switch 1 to switch the signal SYS-1 from the line 12 to line 11 to complete system switching. It again takes about 2 ms for this switching in the STATION A. Thus, it takes altogether approximately 14 ms for the system to switch from a working line (line 12 in this example) to the stand by line 11.
Although rare, sometimes STATION B fails to switch the line. FIG. 3(b) shows the sequence of such a case. The triggering of the switching is similar to that of FIG. 3(a). However, in the STATION B, when the CSC 4 is notified by the LP switch 2 that the switching has failed due to some failure of equipment or parts, the CSC 4 sends a signal indicating that the switching has failed to the STATION A, in the status signal indicating the switching situation. Then, the CSC 3 sends a command to retry the switching to the STATION B. By this command, the STATION B again tries the switching of lines 12 and 11. If the switching is successful, the following step is the same as that of FIG. 3(a). Thus, it will take about 26 ms total for the switching. But if the switching fails again, the CSC 4 sends a signal that the switching has failed and when this signal is received, the CSC 3 stops the switching operation and alarms the operator that the situation is a matter of importance. As has been described, it is important to reduce the switching time to reduce the loss of time in the system.